Until recently, it has not been possible to develop criteria to distinguish between the normal loss of skeletal mass with age and that occurring in osteoporosis. With the use of total-body neutron activation analysis, it has become possible, for the first time, to measure total body calcium (TBCa), directly, and hence to measure skeletal mass. This technique, along with photon absorptiometry, will be used to measure TBCa in a cross-sectional study of normal white and black individuals. The normal skeletal mass can be predicted for an individual on the basis of sex, age, height and lean body mass. Thus the difference between the measured and the predicted value can be determined for any individual. The basic assumption underlying the measurement of bone mass for the evaluation of osteoporosis is that there exists a relationship between the level of bone mineral and osteoporosis. If the hypothesis that the structural integrity of the skeleton is associated with a critical level of mineral is accepted, it is essential to determine this threshold value and to establish the degree of osteoporosis in terms of the deficiency of calcium from a "normal" value. A prospective study of population of post-menopausal women will be initiated to identify a sub-group with low skeletal mass, and hence one which is considered to be at high risk for developing osteoporosis. The effect of prophylactic estrogen supplementation to this sub-group will be evaluated in terms of bone mass as well as in terms of the reduction in the incidence and severity of osteoporosis. Evaluation will be made of the combined effectiveness of growth hormone and calcitonin for the treatment of osteoporosis. Finally, the effect of physical activity on skeletal mass will be studies in a geriatric population